Wireless local and personal (WLAN/WPAN) area networks are rapidly increasing in homes, enterprises, and public access environments. A number of access technologies, commonly known as Wi-Fi technologies, have arisen to meet this demand. Wi-Fi technologies can support diverse applications, including voice over internet protocol (VoIP), high quality video, and Internet access. IEEE Standards, such as the IEEE 802.11 group in the United States, has developed various standards to promote the efficient use of the wireless spectrum and the inter-operability between wireless access equipments from different vendors.
Homes, enterprises, and public access environments have different application requirements. A mix of high-bandwidth multimedia applications, including video, may predominate among home applications, whereas voice, email, and data applications may be more typical of the type of traffic found in enterprises. As the amount of traffic increases on wireless LANs, high-priority and real-time traffic can become less reliable because nodes in the wireless network contend with one another to determine which node can commence transmission. Consequently, quality of service (QoS) mechanisms are necessary to ensure reliable delivery of high-priority and real-time transmissions.
The 802.11e draft standard provides two mechanisms for supporting QoS, called Enhanced Distributed Channel Access (EDCA) and HCF Controlled Channel Access (HCCA). The standard divides the access time to the wireless medium into contention free periods (CFPs) and contention periods (CPs). The HCCA functions provide controlled channel access during contention free periods, whereas the EDCA functions provide contention-based access during contention periods.
During a contention free period, a point coordinator (e.g., an access point) controls access to the wireless channel through polling. The polling mechanism of HCCA ensures a minimum bandwidth to each node that requests to be added to a polling list managed by the point coordinator. The point coordinator polls each node, one at a time, in order to grant that node permission to transmit on the wireless channel, thus avoiding contention between transmitting nodes. A contention period begins after the point coordinator polls every node on the polling list.
During contention periods, nodes gain access to the wireless channel through contention resolved in accordance with a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithm. Between contending traffic flows, EDCA gives priority to nodes transmitting high-priority traffic. For example, a node offering high-priority VoIP traffic has an advantage during contention over a personal computer that is downloading a file (i.e., low-priority data traffic). A disadvantage of EDCA, however, is that high-priority traffic can effectively prevent low-priority traffic, such as data traffic, from gaining access to the wireless medium. Although data traffic is generally tolerant of delay, in order to minimize the number of TCP timeouts and to meet user expectations there should be a bound on the length of this delay.